Conventionally, regarding a vehicle occupant protection apparatus protecting the occupant of the vehicle, a technique is disclosed in a form of an air bag system disclosed by, for example, Japanese Patent Laid-Open No. JP-A-2004-255911 (US20040164534). The air bag system has a switching element for safety and a switching element drive circuit as well as an IC and a squib. The IC consists of a LO side switching element and a LO side drive circuit with a HI side switching element and a HI side drive circuit. The switching element for safety, the HI side switching element and the LO side switching element are connected in series. In addition, the switching element for safety is connected to a power supply. Furthermore, between the HI side switching element and the LO side switching element, the squib is interposed.
There are air bag systems, in some cases, having plural squibs. When plural squibs are used, plural ICs having squibs connected thereto are connected to one switching element for safety. In addition, a capacitor for the noise rejection use is connected to the connecting point of the switching element and the IC for safety to remove a noise emitted from the IC to the outside of the IC.
The switching element for safety is driven by the switching element drive circuit, and the voltage of the power supply is converted into the predetermined voltage to be supplied to each of the ICs. When the HI side switching element and the LO side switching element of a certain IC become an ON state at the same time, an ignition electric current flows to connected squibs. The ignition electric current is intercepted afterwards when the HI side switching element and LO side switching element become an off state together. Then, the electric current flowing through the circuit flows into a capacitor for noise rejection, and the capacitor is charged. Therefore, the voltage of the connecting point of the switching element for safety and the IC suddenly rises. This state will be maintained for a while, because there is no electric discharge route. Upon having the above state, the switching element drive circuit drives the switching element for safety to lower the voltage of the connecting point. When the HI side switching element and the LO side switching element of another IC become an ON state together, an ignition electric current flows to connected squibs. However, the switching element drive circuit is driving the switching element for safety to lower the voltage of the connecting point. Further, because the capacitor is for noise rejection use, the capacity of the capacitor is small, and only a slight electric charge is accumulated even after the voltage rise by charging. Therefore, it is possible, in some cases, that a sufficient amount of electric current is not provided for the squib.